1. Field of the Invention
This invention relates to a system for discriminating firing or ignition state for internal combustion engines that can discriminate firing or ignition state based on ionic-current occurring upon combustion of air-fuel mixture in the engine combustion chamber, particularly to such a firing state discrimination system for internal combustion engines that can accurately discriminate the firing state including occurrence of misfiring, fouling (smoldering) of a spark plug caused by deposits such as soot, carbon residues, etc. and a failure in the ignition system or in fuel supply system based on the ionic-current and leakage current.
2. Description of the Related Art
In a gasoline or other spark-ignition internal combustion engine, a high voltage generated by the ignition coil is applied through an ignition distributor or the like to spark plugs installed in the individual cylinders. The spark discharge that the high voltages produces across the gap between the spark plug electrodes ignites the air-fuel mixture, causing firing and combustion. However, when certain causes are present during the engine ignition/combustion stroke, the combustion of the air-fuel mixture does not proceed normally, i.e., misfiring occurs.
Causes of misfiring fall in two classes, those attributable to the fuel supply system and those attributable to the ignition system. Misfiring attributable to the fuel supply system is the result of either excessively lean or excessively rich air-fuel mixture. In this case, a spark discharge is produced across the gap of the spark plug but the air-fuel mixture does not ignite. Misfiring attributable to the ignition system is the result of spark plug fouling (smoldering) caused by deposits of soot, carbon residues, ash from fuel and oil additives, etc., or of a problem in the ignition circuit that prevents normal spark discharge (mis-sparking).
When the air-fuel mixture burns normally, the combustion is accompanied by ionization of the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture) that gives rise to ionic-current. When misfiring occurs and the air-fuel mixture does not burn, the air-fuel mixture does not ionize and no ionic-current arises.
FIG. 7 shows the ionic-current waveforms during misfiring and normal combustion, in which the discharge is produced by a high voltage of negative polarity. As shown, the ionic-current waveform during normal combustion, i.e., when ions are produced, spikes instantaneously in the minus direction just after the discharge across the spark plug electrodes (as seen at A in the drawing), thereafter continues to flow in proportion to the volume or number of ions produced, and eventually returns to a given level. During misfiring, i.e., when ions are not produced, the waveform spikes instantaneously in the minus direction just after the end of discharge (as seen at Axe2x80x2 in the drawing) and then immediately returns to the given level.
A widely used method of detecting misfiring has therefore been to detect the ionic-current (current waveform) occurring during the combustion stroke using the spark plug, more exactly the electrodes thereof, as a probe for detecting ionic-current, and comparing the detected value with a prescribed value, as taught by, for example, Japanese Laid-open Patent Application No. Hei 5(1993)-99956.
It should be noted that in the figure, spikes marked by A, Axe2x80x2, B and Bxe2x80x2 are sharp rises instantaneously caused by inductive noise due to the electromagnetic induction of spark plug.
The resistance between spark plug electrodes are almost infinity during normal condition. Accordingly, when misfiring occurs and no ions are produced, current does not flow across the electrodes. However, if the aforesaid spark plug fouling (smoldering) occurs, the resistance between electrodes drops to a level of several Mxcexa9 or thereabout, resulting in leakage current flow even during misfiring where no ions are produced. This leakage current may erroneously be detected as the ionic-current, which makes accurate misfiring detection impossible.
When a spark plug becomes fouled (smoldered), no spark charge can jump across its electrodes and misfiring occurs. When this happens, unburned fuel may disadvantageously damage a catalytic converter when post-ignited or may disadvantageously degrade emission. Therefore, it has been desired to accurately discriminate firing or ignition state including spark plug fouling (smoldering) and misfiring, etc., and to inform the result of discrimination to the vehicle operator.
Aside from the above, the supply of fuel is temporarily cut off at certain vehicle running conditions so as to improve fuel economy or some other reasons. When the supply of fuel is cut off, since the fuel injection amount is made zero or thereabout (i.e., the air/fuel ratio is made an excessive lean value), no combustion occurs and hence no ionic-current flows. This is a kind of misfiring state, but should be distinguished from ordinary misfiring which happens unintentionally to the vehicle operator. However, the prior art may disadvantageously detect this state as the ordinary misfiring.
A first object of the present invention is to overcome the aforesaid problems and to provide a firing state discrimination system for internal combustion engines which can discriminate the firing or ignition state of a spark plug including the occurrence of spark plug fouling (smoldering) and can distinguish the misfiring state due to fuel cutoff from ordinary misfiring.
A second object of the present invention is to provide a firing state discrimination system for internal combustion engines which can inform the result of discrimination to the vehicle operator so as to make it possible to prevent degradation of emission.
For achieving the first object, this invention provides a system for discriminating firing state for an internal combustion engine having a spark plug, installed in a combustion chamber of a cylinder of the engine and connected to an ignition coil, which produces spark discharge when supplied with discharge current from the ignition coil to ignite air-fuel mixture in the combustion chamber; comprising: ionic-current detecting means for detecting ionic-current that flows during combustion of the air-fuel mixture, at a predetermined first timing; leakage current detecting means for detecting leakage current that flows across electrodes of the spark plug, at a predetermined second timing which is later than the predetermined first timing; and firing state discriminating means for discriminating firing state of the engine based on outputs of the ionic-current detecting means and the leakage current detecting means.
For achieving the second object, this invention provides the system which further includes: informing means for informing the discriminated firing state to a vehicle operator.